Gas conduit with acoustic insulation comprising anisometric compressed and bonded multilayer knitted wire mesh composites

ABSTRACT

Gas conduits with acoustic insulation are provided which comprises anisometric compressed and bonded knitted wire mesh composites composed of a plurality of sheets of knitted wire mesh, superimposed at random orientation with respect to each other, compressed or densified to a voids volume within the range from about 10 to about 90 percent, and bonded together. The sheets are taken in sufficient number, usually at least five and preferably 10 or more, and as much as 1,000 or more, to form a self-supporting relatively non-resilient composite of high tensile strength and high breaking strength having an average pore diameter of less than 200 microns, and preferably less than 100 microns, that is relatively uniform in any unit area of the surface, and having an anisometric porosity, the through pores extending crosswise of the sheet greatly exceeding in number the through pores extending laterally of the sheet, which latter pores can be reduced vitually to zero in a highly compressed composite. The composite is formed by superimposing a plurality of knitted wire mesh sheets, annealing the composite to avoid wire breakage during later processing, compressing the composite to the desired density and anisometricity by application of pressure in a direction approximately perpendicular to the plane of the layers of the composite, and bonding the sheet layers and the wire filaments of the sheets together at their points of contact and/or crossing. The bonding holds the composite at the selected density, prevents relative movement of the wires in the composite, and in conjunction with the multilayer structure imparts the self-supporting nonresilient characteristic, together with high tensile strength and high breaking strength.

United States Patent 11 1 v Pall GAS cONDuIT WITH ACOUSTIC INSULATIONCOMPRISING ANISOMETRIC COMPRESSED AND BONDED MULTILAYER KNITTED WIREMESH COMPOSITES David B. Pall, Roslyn, Estates, N.Y. Assignee: PallCorporation, Glen Cove, Filed: Se t. 29, 1970 Appl. No.: 76,591

Related US. Application Data Division of Ser. No. 732,443, May 27, 1968,Pat. No. 3,690,606.

[75] Inventor:

References Cite d I UNITED S ATE PATENTS 3/1964 Wheeler .L 181/713,353,626 11/1967 Cremer ct 31.... I 181/42 3,439,774 4/1969 Iallaway etal. 181/71 X FOREIGN PATENTS OR APPLICATIONS 1,513,952 l/l968. France...181/50 Primary Exnminer -Robert S. Ward, Jr. M

57 ABSTRACT Gas conduits with acoustic insulation are provided 7/1955Great Britain 181/42 v l l 3,795,288 I451 Mar. 5, 1974 which comprisesanisometric compressed and bonded knitted wire mesh composites composedof a plurality of sheets of knitted wire mesh, Superimposed at randomorientation with respect to each other, compressed or densitied to avoids volume within the range from about 10 to about 90 percent, andbonded together. The sheets are taken in sufficient number, usually atleast five and preferably 10 or more, and as much as 1,000'01' more, toform a self-supporting relatively non-resilient composite of hightensile strength and high breaking strength having an averagepore'diameter of less than 200 microns, and preferably less than 100microns, that is relatively uniform in any unit area of the surface, andhaving an anisometric porosity, the through pores extending crosswise ofthe sheet greatly exceeding in number the through'pores extendinglaterally of the sheet, which latter pores can be reduced vitually tozero in a highly compressed composite. v The composite is formed bysuperimposing a plurality of knitted wire mesh sheets, annealing thecomposite to avoid -wire breakage during later processing, compressingthe composite to the desired density and anisometricity by applicationof pressurein a direction approximately perpendicular to the plane ofthe layers of the composite, and bonding the sheet layers and the wirefilaments of the sheets together at their pointsof contact and/orcrossing. The bonding holds the composite at the selected density,prevents relative movementof the wires in the composite, and inconjunction with the multilayer structure imparts the self-supportingnonresilient characteristic, together with high tensile strength andhigh breaking strength.

' 17 Claims, 24 Drawing Figures PAIENTED 3,795,288

sum u 0? a SREET 5 OF 8 PATENTEI] HAR 51974 PATENTED MAR 51974 SHEET 60F 8 ms lg 7 at S'IAVH GAS CONDUIT WITH ACOUSTIC INSULATION COMPRISINGANISOMETRIC COMPRESSED AND BONDED MULTILAYER KNITTED WIRE MESHCOMPOSITES This application is a division of Ser. No. 732,443, filed May27, 1968, now US. Pat. No. 3,690,606 patented Sept. 12, 1972.

Woven wire mesh have been in use for some years as filter materials.They have the advantages of being readily available, permitting closecontrol of uniformity in the number, size and shape of the pores, and intensile strength,,as well as being adapted for fabrication and beingrelatively low in cost. Various forms of such materials have beenprovided, ranging from the woven wire mesh as commercially available, towire mesh specially treated so as to better suit them for filter uses.

US. Pat. No. 2,423,547 to Behlen, dated July 8,

1947, suggests rolling a wire mesh to form a flat sheet,

' fore it is effectively clogged is referred to as the dirt capacity ofthe filter, and this can be measured in various ways. For referencepurposes, it is usually expressed in terms of grams of standardized dirtper unit surface area of the filter as determinedby a standardized testprocedure.

US. Pat. Nos. 2,925,650 and-3,049,796 to Pall describe and claim wovenwire sheet material specially treated by sinter-bonding, with a slightor great deformation of the wires at their points of crossing, whichpossess several advantages over the Behlen material. Not only are thewires held against a relative shift in position during treatment,because of the sintering operation, but the material also retains muchof the nature of the starting wire mesh, and therefore much if not all'Filter media can generally be classified as being oneof two types,depth filters-and surface filters. A depth filter removes suspendedmaterial'from the fluid passed through the filter by collecting it notonly on the surface of the element but also within the pores..A depthfilter has a considerable thickness, and has'a plurality of-pores ofdistinct length. The longerthe pores, the higher the dirt capacity ofthe filter, because there is more room for dirt along thepores. Mostdepth filters are made of masses'of fibers, or other particulatematerial, held togetherby mechanic al means or by bonding. One orseveral layers of such materials can be employed,'and these layers canvary in porosity. In most cases, however, the greater percentage ofcontaminants unable to pass through the filter is trapped at the surfaceof the filter.

A surface filter removes suspended material from the fluid passedthrough the filter by collecting such material on its surface, and thematerial thus removed forms a filter cake or bed up'onthe filter. Thismaterial natu rally obstructs the openings in'the surface of the filter,

because the fluid must flow through this material,

which thus effectively reduces the diameter of the filter openings tothe size of the pores in the filter cake. This reduction in effectivediameter of pore openings in the filter increases the pressuredifferential required to maintain flow through the filter.

Woven wire mesh filters of the square weave type fall in the category ofsurface filters, because the depth of the pores through the sheet issubstantially no greater than the diameter of the filaments making upthe weave. Consequently, these filters have a rather limited dirtcapacity, as compared to depth filters. US. Pat. No. 3,327,866 to Pallet al. describes woven wire mesh which, by an appropriate selection ofwire size and wire count, in both warp and shute, is formulated tospecified pore size Dutch twill weaves of extraordinarily high dirtcapacity, as compared to Dutch twill weave wire mesh woven of wires ofother sizes and/or counts.

Knitted wire mesh filter elements have been known for many years.However, the physical properties of knitted mesh made of fine diameterwires are such to defy any modification previously attempted to renderthem suitable as filters forlariything other than coarse filtration ofliquidsand gases, such as air, since they'"have lacked a reliable poresize uniformity and their maximum pore size has been rather high, wellin excess of 200 microns. i

A number of patents have described ai'r filters made in whole or in partof knitted wire mesh, among them, No. 1,676,191 to Jordahl, No.1,905,160 to De Ange lis, No. 1,829,401 to Ka'mrath, No. 2,274,684 toGoodloe, No. 2,327,184 to Goodloe, No. 2,334,263'to Hartwell, No.2,439,424 to Goodloe et al., No. 2,462,316 to Goodloe, No. 2,672,214 toGoodloe, No. 2,792,075 to McBride et al., No. 2,929,464 to Sprouse, andNo. 3,085,381 to Sobeck. Virtually all of the filter elements thusproposed comprise a plurality oflayers of knitted wire mesh. However,the problem presented by knitted mesh is best summarized by Goodloe inNo. 2,327,184: 7 a

Although it is highly desirable from the standpoint of efficiency toemploy layers of fine mesh, such fine mesh, especially when of knittedcharacter, is generally of flimsy character, is not self-sustaining, andconsequentlya filter body composed of layers thereof is easily subjectto compression by the force of the air or gas stream movingtherethrough, whereby tendency to crowd the layers together isincreased, so that the undesirable conditions above referred to arefurther enhanced. Y

Goodloe and the other workers in the field have resolved these problemsas best they could, using reinforcing spacers (as in No. 2,327,184), bycrushing or compacting the plural layers endwise or crosswise (as in No.2,439,424, No. 2,462,316 and No. 2,672,214) or by supporting them withina filter unit frame (as'in No. 1,676,191, No. 1,829,401, No. 2,792,075,No. 2,929,464 and No. 3,085,381). S'uchexpedients are acceptable for gasfilters, but they arc'not capable of overcoming the flimsy,nonself-supporting nature of knitted wire mesh to render them suitablefor liquid filtration, where the fluid pressures are considerablyhigher, and-where high strength combined with low or minimal flowresistance are indispensable prerequisites.

One of the outstanding characteristics of a knitted material, as opposedto a woven material, is its resil- A knitted wire mesh can be stretchedover I percent in any direction, despite the nonresiliency of the wirefilaments of which it is made up. In an air filter, this inherentresiliency (which is due to the looping of the filaments in the knitweave) is an advantage, as Jordahl pointed out'in No. 1,670,191, and italso makes it possible to fold and crush the material in any direction.as in Nos. 2,439,424 and 2,462,316. In a liquid filter, however, thisresiliency is a disadvantage, since it means, in effect, that pore sizevaries with pressure drop across the filter, the pores of the materialclosing as the pressure drop increases.

It is equally evident that if the filaments be locked in position, bybonding or other means, the resiliency is not diminished appreciably,because the looped condition of the filaments is unaffected by thebonding. indeed, Jordahl pointed out that even when a plurality oflayers of knitted mesh are superimposed, and compressed to any desireddensity, a relatively great density may be obtained without danger oflosing the characteristic indicia of the structure above described,since the uneven or irregularly roughed surfaces of the fabric, due tothe multiplicity of interlocked strand loops distributed thereover, willalways tend to produce sufficient separation of the component folds,sheets or layers as to assure the requisite low air pressure resistanceof the structure. It was because ofthis that Goodloe in No. 2,274,684used comparatively stiff wire,

- whereby the interengaged loops forming the fabric tend to resistrelative displacement and consequently tend to retain and maintain theinitial shapes and uniform distribution of the loop defined openingsthroughout the area of the fabric, as well as a considerable degree ofself-supporting stability due to inherently greater resistance to bothcontraction and elongative stretch-of the fabric;. Because of thesedifficulties, which are not found in woven wire mesh, and becauseknitted wire mesh presents no apparent advantage over woven wire mesh,whether in one or in several layers, knitted wire mesh has not beenpreferred to woven wire mesh in liquid filters.

In accordance with the invention, knitted wire mesh is formed into aself-supporting relatively rigid anisometric multilayerstructure that isnot only eminently suited for use in acoustic absorption, but also hasunexpected and advantageous properties as compared to similar materialsmade of woven mesh. The knitted mesh material of the invention has lowerflow resistance than comparable woven mesh material of the same poresize, and it is also stronger, is more uniform in permeability, has ahigher modulus of elasticity v( Youngs Modulus), usually at least 3.3percent of the modulus of solid sheet of the same material, andfrequently much higher, and when materials of equal particle removal arecompared, has a higher dirt capacity. For stainless steel this value isat least l X psi. Why this is so is not at present understood, and noexplanation thereof can be offered, but the superiority is clearly to beseen in the data that has been collected, of which a representativeselection is given in the examples. l

The knitted wire mesh of the acoustic insulation in the gas conduits ofthe invention comprises a plurality of sheets of knitted wire mesh,superimposed at random orientation with respect to each other,compressed or densified to a voids volume within the range sometric, thenumber of through pores extending across the sheet exceeding the numberof through pores extending laterally of the sheet. The number of lateralthrough pores can be reduced virtually to zero, if the'degree ofcompression or densification is great enough, and this can be anadvantage in many uses. The thickness need not be great, provided thesheet is self-supporting, and preferably is within the range from about0.001 to about 05 inch.

The composite is formed by superimposing a plural-- V 'ity of knittedwire mesh sheets, preferably (but'optiom ally) annealing the compositeto avoid wire breakage during later processing, compressing thecomposite to the desired density by application of pressure in adirection approximately perpendicular to the plane of the layers of thecomposite, and bonding the sheet layers.

and the wire filaments of the sheets togetherat their points of contactand/or crossing. r

The annealing softens the wires which are work hardened as a result ofthe knitting operation, and permits them to bend or deform duringcompression without breaking. While it is preferable to anneal thecomposite to reduce annealing cost, annealing of the knit mesh sheetsbefore they'are superimposed into a composite is an equivalent step,serving the same purpose.

If the composition of the wire is such that very little work hardeningoccurs during knitting, the annealing step may be omitted.

The bonding holds the composite at the selected density, preventsrelative movement of the wires in the composite, and in conjunction withthe multilayer structure imparts the high modulus of elasticity, usuallyat least 1.1 X 10 psi., together with high tensile.

strength and'high specific'strength.

In a preferred embodiment of the invention, the filaments of the knittedwire mesh are sintered to integrate them, at the bonding stage of theprocess. The filaments can be integrated by sintering prior tocompressing, so that they no longer are able to shift their relativepositions. The sintering process also anneals the filaments. However,since in the stack of knitted mesh sheets the wires are sufficientlystable, against relative movement during compressing, being held inposition by the interlocked loops of wires of adjacent sheets, they maybe compressed, preferably by rolling, before sintering.

The rolling and sintering can be repeated as many times as desired tomeet any desired porosity and density requirement. In certain cases, theeffect ofa rolling 1 utilized in the process to provide strongself-supporting relatively nonresilient anisometric knitted very thinmultilayer composite sheet products having a relatively large number ofpores in a uniform pattern, and a uniform porosity across but notlaterally of the sheet.

This high voids volume, strength, good particle removal rating, andanisometrically disposed uniform pore characteristics of the knittedwire mesh compostending chambers which are dimensioned to damp orattenuate sound waves of the selected wavelengths and which may or maynot contain sound absorbing material. The knitted wire mesh compositesare superior to perforated metal sheets, sintered woven wire mesh, andsintered non-woven wire mats of comparable pore size and thickness, asthe surfacing material in suchinsulation f The anisometric knitted wiremesh'c'omposites are also susceptible of being made with a relativelylow voids volumegusing'knitted mesh of relatively large wires and smallpores, and a relatively high thickness, so that they are especiallysuited for use in transportation cooling, when they can serve as porouswalls or wall liners for passage of cooling gases to efficiently removeheat from the chamber walls, as in jet engine combustion chambers,rocket engine fuel injection systems, turbo-jet blades, and the surfaceskin of hypersonic aircraft, reentry and aerospace craft,.and otherspace vehicles and projectiles. i

By use of the knitted mesh composites, it is possible to make poroussheets from wire materials which cannot be woven into suitable wiremesh. If a wire mesh is not to be too sleazy (i.e. if the wires are notto befree to slide around), the wires must be elongated and deformedtosmall radius during weaving. The most common woven meshes (e.g. 325 X325 X 00014, 200 X 200 X 0.0021) require wire with an elongation of atleast 25percent, and very few weaves can be made from wires havingelongations of less than-l5 percent. By contrast, wire materials withvery low elongations can be knitted, and then used to make thecomposites. Several knitted wire mesh composites, and acousticinsulation containing'the same, all in accordance with the invention,are described in detail below, having reference to the embodiments shownin the accompanying drawings, in which:

FIG. I is a plan view of a rigid warp-knitted mesh composite, showing intwo parts the starting mesh and the composite, respectively; 1

FIG. 2 is a view in transvers'e section taken along the.

line 22 of FIG. I, and looking in the direction of the arrows;

FIG. 3 a photograp'hic top view enlarged six times ofaloose-weft-knitted mesh stack, ready for compressing and bonding to forma composite of the invention;

FIG. 3A is a photographic top view enlarged six tiems of a weft-knittedmesh composite, made from the mesh stack of FIG. 3.

I FIG. 3B is a photographic end view of the composite of FIG. 3A;

FIG. 4A is a photographic top view enlarged six times of another knittedmesh composite; I

FIG. 4B is a photographic end view of the composite of FIG. 4; 7

FIG. 5 is a plan view (with the top layer partly broken away) of atubular loose-weft-knitted mesh composite made of mesh of differentneedle ends, showing the starting mesh in one part and the composite inanother part;

FIG. 6 is a view in transverse section, taken on the line 66 of FIG. 5,looking in the direction of the arrows;

FIG. 7 is a plan view of a flat close-weft-knitted mesh composite,showing the starting mesh in one part and the composite in another part;

FIG. 8 shows the apparatus used to determine Rayl number (gaspermeability) of a knitted wire mesh composite;

FIG. 9 is a graph of air flow (standard cubic feet per hour per squarefoot) against Rayl number for a knitted wire mesh composite;

FIG. 10 shows anapparatus for determining the normal incidenceabsorption coefficient and inductive reactance of a knitted wire meshcomposite;

FIG. 11 is a graph of sound pressure against distance for the sample ofknitted'wire mesh composite;

FIG. 12 is a cross-sectional view showing a gas duct with a honeycombacoustic insulation structure incorporating as the front face a knittedmesh composite;

FIG. 13 is a cross-sectional view showing a gas duct with anotherembodiment of honeycomb acoustic insulation structure, including theknitted mesh composite;

FIG. 14 is a crosssectional view showing a gas duct with anotherembodiment of acoustic insulation structure, including the knitted meshcomposite;

FIG. I5is a cross-sectional view showing a gas duct with anotherembodiment of acoustic insulation structure, including the knitted meshcomposite;

FIG. 16 is a cross-sectional view showing a gas duct with anotherembodiment of acoustic insulation structure, including the knitted meshcomposite;

FIG. 17 is a cross-sectional view of another embodiment of acousticinsulation in honeycomb form, including two faces of knitted meshcomposite;

FIG. 18 is a cross-sectional view of another embodiment of acousticinsulation'in honeycomb form, with one face of knitted mesh compositeand one face of a perforated plate; t

' FIG. 19 isa cross-sectional view of a knitted mesh compositesinter-bonded to a woven wire mesh;

. FIG. 20 is a cross-sectional view of a knitted mesh compositesinter-bonded to a perforated metal plate;

FIG. 21 is a cross-sectional view of a knitted mesh compositesinter-bonded to a metal plate.

A knitted mesh is composed of rows ofloops, each caught into theprevious row, and depending for its support on both the row above andthe'row below. There are two types of knitting, weft and warp. Inweft-knit mesh the loops run crosswise of the fabric, and each loop islinked into the-loop on the preceding row. In warp-knit mesh, parallelyarns are united in a chain stitch, first one yarn'and then the otherzig-zagging to tie the yarns together; and the loops interlock bothweftwise and warpwise in the fabric. Warp-knitted mesh has about fourtimes as many stitches to the inch as weft-knit mesh, and is of astronger and closer construction.

When knitted mesh layers are superimposed, at random, the pores ofadjacent mesh layers do not necesblock laterally extend- This relativedisplacement is consequently an advantage, since it has the effect ofreducing the size of the through pores in the composite. Loops ofadjacent mesh layers project into and partially obstruct the pores ofthe next adjacent layers, and upon compression this effect can berepeated many times, with loop wires from layers as far as three or fourlayers away joining in this obstruction. .Thus, in a composite of fiveto ten layers, through crosswise pores can be reduced to as low as anaverage of 50 microns or less, using mesh having an initial 500 to10,000 micron pore size.

At the' same time, these projecting loops selectively block thelaterally-extending through pores to a greater extent than the crosswisepores; and this effect is increased as the number of layers and thedegree of compression or densification is increased.

The result is an accentuation of the a-nisometricity of the startingknitted mesh, tov the point where the through'por'es that extendlaterally can be blocked altogether, and thereby extinguished.

. The tortuousness of the throughpores in these composites is' incontrast to the pores through a woven wire mesh, such as a square weavemesh, which. are of the straight-through variety, or of a Dutch twillweave material, which are angled.

The knitted wire mesh composites can be made up of warp-knitted orweft-knitted wire mesh in any combination of mesh, wires, pore sizes,and knit types and stitches, such as plain stitch or purl stitch, flatstitch or rib stitch, open. work stitch or tuck stitch, weft-knit mesh;and single-bar tricot, double bar tricot and Milanese warp-knit mesh.Flat knit and circular knit mesh can be used.'Circular knit mesh can becut open, or used-double. I

The knitted mesh composites can be knitted of wires of any metal. Foracoustic uses, metals which are inert to and non-corroded by the fluidbeing filtered or the gas in contact therewith are of course preferred.Stainless steel is a very suitable material. Aluminum, brass These canbe knitted using conventional textile knitting machinery to mesh of therequired needle ends, or

loops per inch, wire diameters, and pore sizes. in general, the meshshould not have more than about 30 needle ends per inch, but there is nolower limit. If the knitted mesh is rather o'pen, i.e., if the needleends are only 2 per inch, or less, more layers may be needed to re duce'pore size to the desired maximum than if closer knitted mesh is used,but if large initial thickness of the composite is not a factor, this isnot a disadvantage.

The wires are usually monofilaments. Wires less than 10 mils indiameter,.and preferably from I to 5 mils in diameter, are preferred forfilter uses. The wires can be of any cross-sectional configuration, suchas round, square, fiat, polygonal, elliptical and rectangular. Strandedmultifilament wire can be used.

It is sometimes advantageous for some filter uses to use magnetic wires,or to interleave mesh of magnetic wires with mesh of non-magnetic wires,in the mesh composites. in some cases, it may be useful to alternatemesh of magnetic wires with mesh of non-magnetic wires. l

The composite is prepared by superimposing a selected number of knittedwire mesh sheets, one above the other. The orientation is random,preferably, since this best enables each sheet to remedy anynonuniformity in the next sheet, and produce a composite that is uniformthroughout, but an orderly or a patterned orientation, such as layingalternatcsheets at right angles, or'other specific orientation, to theone below may have advantages in some instances.

If the knitted wire mesh have not previously been annealed, thecomposite is preferably annealed first, to I soften the wire filaments.This is especially desirable when the wire filaments are 4 mils or lessin diameter. Annealing is at a temperature and for a time appropriatefor the metal of which the filaments are made, and

is usually at from about 1 50 to about i ,l 25C. for from 10 minutes to48 hours. The following are exemplary:

- Temperature Time Metal (C.) Minutes) Stainless Steel 1000-l I25 l(l3()Copper 260-650 l0-30 Steel 810-875 l(l30 Monel 875-l000 l0-30 Aluminum350-4l5 Hl-litt) pendicular, since the displacement component may beexcessive at such larger angles. The compression can be applied withrestraint, as in a mold, but it is preferably without restraint, as byplatens, or by pressure rollers. Rolling is preferred;

The composite should be subjected to a pressure of the order of to200,000 lbs. per square inch, the pressure applied depending upon theductility of the metal, and applied normal to the metal surface, as by Vrolling or coining. lf thepressure is less than the deforming pressurefor the metal of the wires, usually about 50,000 lbs. per square inch,it merely results in densification of the composite, by forcing thelayers and the wires closer together.

If the pressure applied is sufficiently great, a coining action can beobtained, in which the composite is compressed to as'little as about .10percent of the starting thickness. Reductions of as little as 30 percentin the erably the reduction is tofrom about 30 percent to about 65percent of the starting thickness.

After compression, the layers and the filaments are set in their newrelative positions by bonding them together at their points of contact.The layers andfilaments can if desired be bonded by welding, brazing,soldering or sintering, or by use of resinous bonding agents, applied assolutions, dispersions, or from a fluidized bed of the resin. They will,of course, be mechanically interlinked or interleaved or interlocked, as

starting thickness can be sufficient, however, and prefstance in US.Pat. Nos. 3,246,767 and 3,353,682.

A structure particularly useful in acoustic insulation is a honeycomb inwhich a knitted mesh composite forms one face, and the other face is aporous or nonporous material such as another of the same knitted meshcomposite, woven wire mesh, perforated sheet, or

imperforate sheet. The two faces enclose the cellular a result of thecompression, so that a very strong structure results. I

Brazing, soldering, resin bonding and welding, while fully satisfactory,may reduce porosity and pore size to an undesirable extent.Consequently, it is frequently preferred to integrate the filaments attheir points of contact by sintering.

' The composite can be sintered by passing it through a furnace in anon-oxidizing atmosphere, such as, for example, in a reducing atmosphereof hydrogen or carbon monoxide, or mixtures thereof; or in an inertatmosphere such as nitrogen, argon, helium, or combinations thereof; orin a vacuum. The mesh is heated to a temperature not exceedingapproximately below the melting point of the metal of which thefilaments are formed. Generally, the temperature will be in excess ofl,000 F. The result is a sintered integration of the metal at the pointsof crossing.

After bonding has been effected, the composite can be compressed again,such by rolling, and then bonded again, such as by sintering, and thesesteps can be repeatedas many times necessary to give a composite havingthe desired characteristics, for filtration, for acoustic insulation, orfor other uses. The final composite generally will have been reduced tobetween about 10 percent and about 95 percent of the starting thickness,and the pore anisometricity will be such that the permeability for flowthrough the pores extending laterally will be less than about 75 percentof that for flow through the pores extending across the sheet, andpreferably less than 60 percent, and this can be reduced to zero. Thepermeability is defined as the volume of flow of any fluid at unitdifferential pressure through a unit cube; 2 I

As one or several of the juxtaposed layers there can also be used wovenwire mesh, or metal plates or sheets, which can be perforated orimperforate, and

which can be at the surfaces or in the interior, and which can be bondedthereto by any of the procedures indicated above. The combination of theknitted wire mesh composite with a perforated material is particularlyuseful, as it permits the manufacture of light weight high strengthmaterials,useful in sound absorption in airborne applications. A layerof metal powder can be dusted into the knitted wire mesh composite, orsuperposed on one or both surfaces thereof, and bonded thereto, forexample in accordance with US. Pat. No.3,0l7,9l7, dated Nov. 6, I962.

If desired, the knitted mesh wire composites of the invention can alsobe laminated to other materials, such as woven wire mesh, and metalplates and sheets, perforated, if desired. I

The knitted wire mesh composite can also be impregnated and/or coatedwith fibrous material such as inorganic, metallic or organic fibers, asdisclosed for inhoneycomb core structure, made up of cross wallsextending between the faces. These cross walls are nonporous,orimperforate, usually, but they can be porous, as well. Such a structurecan include one or more internal faces or dividers, which can be bondedto the honeycomb core by any of the procedures described above. However,procedures which do not cause substantial blockage of the open area ofthe knitted mesh composite are preferred, such as sintering.

FIGS. 1 and 2 show an anisometrie knitted wire mesh composite, made upof 10 layers 1 of a warp-type single-bar tricot knitted wire mesh, 12needle ends per inch, of 4 mil stainless steel wire 2, rolled andsintered to a voids volume of percent and athickness of 0.04 inch.

FIG. 3 shows photographically, enlarged six times, a stack of 16 layersof loose-weft-knitted mesh, of 0.004 inch diameter wire, ready forrolling and sintering. The looped wires of the mesh retain their knittedidentity quite clearly.

FIG. 3A shows photographically, enlarged six times, the same stack afterrolling and sintering. The knitted pattern is still evident, but theconsolidation of the mesh has resulted in a considerable reduction inthe size of the mesh openings. The end view 38 shows that theconsolidation has in fact resulted in a sheet that is plate-like incharacter, with a smooth surface. The porosity is anisometric, with manythrough pores extending across the sheet, and few through poresextending laterally.

FIG. 4A shows photographically, enlarged six times, another sintered androlled composite, made of 10 layers of the loose-weft-knitted mesh ofFIG. 3. The composite is more open, and the pore size and voids volumeare greater, partly because the number of layers is less,.

and partly because of a lower pressure and less percent reduction inthickness during the rolling and sintering operation. Nonetheless, asFIG. 4B shows, the sheet is platelike in.character, and anisometric inporosity, the porosity laterally being lower .than that across thesheet.

' FIGS. 5 and 6 show another anisometric knitted wire mesh composite,made up of 30 layers of knitted mesh sheet. The first 15 are of aloose-knit weft type stainless steel knitted mesh 4, l2 needle ends perinch, and the second 15 are of aloose-knit-weft type stainless steelmesh 5, l8 needles per inch. Both knitted mesh are made of2 mil wire 3.The difference in needle ends of the two mesh produces a compositehaving coarse 50 microns average pores on the upper side (shown in FIG.6) and fine 10 microns average pores on the lower side.

FIG. 7 shows an anisometric knitted wire mesh composite made of fivelayers 6 of close-knit weft type stainless steel knitted mesh, made of10 mil wire 7.

FIG. 19 shows a knitted wire mesh composite l of the invention, asshown, for instance, that of FIGS. 1 and 2, sinter-bonded to a sinteredsquare weave stainless steel wire mesh 8.

FIG. 20 shows a similar multilayer structure, in which the knitted wiremesh composite l is sinter-bonded to a perforated stainless steel plate9.

FIG. 21 shows a similar multilayer structure in which the knitted wiremesh composite 1 is sinter-bonded to a solid stainless steel sheet 9'.

The voids volume of the anisometric knitted wire mesh composite isdetermined by measuring apparent volume and true volume. The apparentvolume of the material is determined by measurement of its area andthickness. The true volume is determined by fluid displacementtechniques,'using a fluid capable of wetting the product. The voidsvolume is then determined by the following equation:

Voids volume= 100x 1 true volume of composite percent andv in someinstances 80 percent and even higher.

apparent volume of c gnposite] The pore size or diameter of the knittedwire mesh composites is evaluated by the'following test,'which issubstantially in accordance with the procedure of US.

Pat. No. 3 ,007,3 34.

'A disk of the material to be tested is wetted with a fluid, preferablyethyl alcohol, capable of wetting the porous material, and clampedbetween rubber gaskets. The volume above the disk is filled with thefluid. Air pressure is'increased in the chamber below the disk until astream of air bubbles is observed emerging from one point of the testpiece. The effective pore diameter is then calculated by the well-knownformula;

This formula is discussed in WADC Technical Report 56-249, dated May,1956, entitled Development of Filters for 400F. and 600F. AircraftHydraulic Systems" by David B. Pall, and available from the ASTIADocument Service Center, Knott Building, Dayton 2, Ohio. A detaileddescription of the bubble point test and determination of pore size fromthe maximum particle passed will be found in Appendix l of this report.See also US. Pat. No. 3,007,334, dated Nov. 7, 1961, to David B. Pall.

K is determined by measuring the maximum spherical glass bead orcarbonyl iron particle which passes through the element, in accordancewith WADC Technical Report 56-249 and MlL-F-88l5B Paragraph 4.6.2.5(August 10, 1967).

The pore diameter obtained by this method is the maximum porediameterJ-By continuing to increase air pressure until the whole surfaceof the filter medium is bubbling (knownas the open bubble point), thesame constant can be used to compute an average diameter characteristicof most of the pores. Tests have shown that if air is passed at avelocity of 70 to 170 cm/min, the pressure necessary to achieve the openbubble point taken together with the K value given above gives a valuefor the pore opening approximating the true average value. The ratiobetween the maximum pore size and theaverage pore size of themicroporous media of this invention generally ranges from about 2:1 toabout 4:], a relatively small difference tlOIL;

Example 1 Four anisometric knitted wire mesh compositions were prepared,made of 0.001 1 inch diameter AlSl 347 stainless steel wire, using aweft knit mesh that had from 12 to 18 needle ends per inch. Sixteenlayers of this mesh were'stacked at random orientation to make acomposite, and sintered at 1,200C. The composite was cut into fourpieces which were rolled to thicknesses of 0.007 inch, 0.0045 inch,0.003 inch and 0.002 inch, respectively. The four layers were stacked inthat order, and resintered to make the final anisometric composite. V

The dirt capacity of the final composite was determinedin accordancewith the following test procedure, whichrepresents a modification of theprocedure of Military Specification MlL-F-88l5B. The compositedescribed'in the preceding paragraph was clamped-in a flow jig fittedwith gaskets 3% inches OD and 3.06 inches ID, and connected to apressure build-up and collapse pressure apparatus, as defined in Section4.6.2.7 of MlL-F-88l5B, Aug. 20, 1967. Hydraulic fluid conforming toSpecification MIL-H6606 was run through the mesh at a flow of 40 gpmlftDirection of flow was such that the upstream face of the test piece wasthe 0.007 inch (highest voids volume) face. Standardized fine aircleaner (A-C fine) test dust in a slurry was added through the dustvalve in 0.2 gram increments at'four-minute intervals. The clean-upfilter was not by-passed during this test. Two minutes after each testdust addition, the pressure differential at rated flow through theapparatus was recorded. The initial pressure drop was 0.2 psid., and theweight of contaminant added in the same manner to develop a differentialpressure across the mesh of 15, 40 and 90 p'sid. was, respectively, 85,91 and 97 grams/sq. ft. After cleaning the composites, a suspension ofglass beads in oil was passed through them. The maximum bead passed was62 microns. This .is the maximum particle rating.

These data represent a very high dirt capacity, considerably higher thana woven wire mesh of equivalent maximum particle rating. For a 325 X 325X 00014 stainless steel square weave wire mesh, of nominal opening 43microns, and maximum particle rating 51 microns, the weights ofcontaminant (AC Test Dust) were 20, 24, and 26 g./sq.ft. fordifferential pressures of IS, 40, and 90 psig., respectively for a 200 X200 X 0.0021 stainless steel square weave wire mesh of nominal opening74 microns and maximum particle rating 38 microns, the weights ofcontaminant were 56, 63, 68 g./sq.ft. to 15, 40 and 90 psig.,respectively. These two woven meshes are industry standards for removalratings in the range of 43 to 83 microns. Thus, the knitted wire meshcomposite of the invention has a dirt capacity greater than that ofwoven wire mesh of comparable or somewhat larger pore size.

Example 2 dle ends per inch. Ten layers of this were stacked, an-

. 13 ne'ald, r6115? snarsiinerad 51 E200 "CI 16 a'cofii asna 0.006 inchthick. Twenty layers were stacked, annealed, rolled and sintered at1,200 C. to a composite 0.008 inch thick. Tenlayers were stacked,annealed, rolled and sintered at 1,200 C. to a composite 0.0023 inchthick. The three composites were then stacked in that order, andresintered to make the final anisometric composite.

The dirt capacity and maximum particle rating of this wire meshcomposite were determined in accordance with the test procedure ofExample 1. The weights of contaminant to 15, 40 and 90 psig. were 56, 60and 78 grams/sq. ft., respectively.

The maximum particle rating was 71 microns.

Example 3 An anisometric knitted .wire mesh composite was prepared madefrom 64 layers of 0.0011 inch A181 347 stainless steel wire weft knitmesh having 12 to 18 needle ends per inch, stacked, annealed, rolled andsin- TABLE 1 l4 and sintered at l,200C. The resultant composite wasclamped in a flow jig as in Example l with the 0.028 inch thick portionupstream.

The dirt capacity and maximum particle rating of this composite wasdetermined as in Example 3. The weights of contaminant'to 15, 40 and 90psig. were 81, 90 and 97 grams. respectively.

The maximum particle rating was microns.

Example 6 to 19 noted in the table, and then resintered at 1,100

1,250C. The bubble points and air flow at the pressure differentialnoted and the Rayl numbers were determined, and are listed. The Raylnumber is a measure of flow resistance, and is discussed below.

' Bubble point AP At air Wire -Thick- (inches of Water column) (inchesof flo Example diameter N0. of ness Water (feet per Wei ht Rayl No.(inches) layers (inches) 151; 10th Open column) second) (lbs/1t. No.

0.002 122 0. 046 1. 75 1. 80 2. 2 2. 7 10 0.47 10 0. 002 122 0. 026 2.15 2. 65 3. 8 8 10 0. 48 32 0. 002 122 0. 024 3. 1 3. 4 5. 0 12. 10 0.48 50 0.002 242 0.043 3.1 3. 5 3. 9 11. 8 10 0.80 50 0. 002 242 0. 0324. 3 5.0 6. 7 e 2. 7 10 0.80 150 0.002 242 0.027 5. 2 5. 9 10. 4 12 100. 80 450 0. 002 2/12 0. 021 7. 9 10. 8 24. 2 I 10 2. 5 0. 80 2, 250 0.003 56 0.017 1.0 1. 4 2. 6 2. 7 10 0. 45 10 0. 003 70 0. 02A .1. 0 1. 32. 5 3 10 0.52 11 0. 003 152 0. 050 1. 8 2. 0 3. 4 7. 1 10 0. 75 32 '0.008 152 0. 044 2. 0 2. 4 4. 2 12. 1 10 0. 75 50 .0. 004 32 0. 020 1.0 1. 2 2. 2 4.0 10 0.52 11. 5 0. 004 70 0. 038 1. 5 1. 0 3. 8 10.0 10 0.00 0. 004 82 0. 046 1. 8 2. 4 3.7 14. 3 10 1.0 50

151531181266 6. to a composite 0.028 inch thick.

The dirt capacity and maximum particle rating of this composite weredetermined in accordance with Example 1, except that the flow was 50 g.p.m./sq. ft. The weights of contaminant to 15, and 90 psig. were 91-,-107 and 120 g./sq.ft., respectively.

The maximum particle rating was 60 microns.

Example 4 The procedure of Example 3 was repeated except that the finalcomposite was rolled and sintered to a composite 0.018 inch thick.

The dirt capacity and maximum particle rating of this composite weredetermined in accordance with Example 3. The weights of contaminant to15, 40 and 90 psig. were 64, 68 and 80 grams/(t respectively.

The maximum particle rating was 31 microns.

Example 5 The composites of Examples 3 and 4 were stacked The tensilebreaking strengthjspecifie strength (ratio of breaking strength toweight per unit area) and Youngs' Modulus were determined for severalofthe Examples given in Table l. This data is given in Table TABLE 11Example Tensile Breaking Specific Young's Modulus No. Strength, lbs/ft.Strength ft.* psi. 6 3468 7332 4.2 X 10 7 3432 7150 8.1 X 10 8 3744 780010.2 X 10 17 3564 6854 11.4 X 10 18 7140 7933 12.5 X 10 19 8820 882010.3 X l0 For comparison with Examples 1 to 19, similar data is given inTable III for a number of sintered woven wire mesh sheets of comparableweights and pore size, made of the same stainless steel, AlSl 347 asthese examples.

TABLE III Weight Breaking Specific Young's Woven mesh (1bs./ strengthRayl strength modulus sample Type weave I sq. ft.) (lbs/ft.) N 0. (ft.)(p.s.i.)

A sintered Dutch twill 0.46 2,232; 2,760 10 4,862; 6,000 1.3X10 B.-- do0.70 3, 744 34 5, 349 4.6)(10 sintered square weave O. 3, 35 6, 200 4;9X20 D .do 0. 60 3, 360 50 5. 3x10 Directional properties.

i is not very large. This indicates that, for any alloy, the

lighter, for equivalent effectiveness. This is a particu- 'larly usefuladvantage in aircraft, missile and submarine applications.

Examples 36m 39 breaking strength is about proportional to the weight of5 material used, with the amount of compression per-. It 8150 remarkablethat the knitted mesh ompos formed n th t i l h i at most a ll ff itesprovide more uniform permeability over'the f ace'of These anisometricmaterials were satisfactory as fila composite than Woven Wire mesh of mpr ters for air and for liquids, and as acoustic absorption nominalPermeability Four knit mesh Composites and di l four woven wire meshsheets were checked for Rayl number, 16 places each on sheets 18 X 48inches in 7 Examples 20 to 32 size. Results are given in Table Vl. Anumber of anisometric knitted wire mesh composi ites were prepared, fromweft knitted wire mesh made TABLE VI ofAlSl 347 stainless'ste'el wire,0.002, 0.003, and 0.004 Example Average inch in diameter, as noted inTable IV. These were YP y vflrifltifl'n 7/ Vurifllim 3s Knitted 10 i 0.33.0 stacked, using the number of layers noted in the Table, Comm, 5 WM 7I 1 V v sintered at 1.150 l,4()0C., rolled to the thickness 37 Knittednoted in the table (0.0] to 0.05 inch), resintered at l,l()0 |,250C,,and the Ray] number and tensile comm! o Woven 34 L 0 +I7.6 strengthdetermined Knmcd. 50 '2 I TABLE IV Control H Woven 50 :tii

(liar i No. of Rayl thiiit ii In allcases, the percentage variation forthe knitted (Inch) Q layers N (Inch) wire mesh composites was lowerthanfor the woven 3:88: 8 $3 95?? mesh, ranging from 3 to 6.5 percentfor the knitted 0. 003 0.53 94 43 0. 0245 mesh composites as aga1nst'7.7to 17.6 percent for the as: 0. -8.2 23 22:2 m 0. 004 0.72 52 45 0. 02353O 81% 313%. 33 3% 3:83? Example 40 8183i 81% 28 2; 693 An anisometricknitted wire mesh composite was [1002 M5 212 32 10425 prepared made from122 layers of 0.002 inch AISI 347 0. 002 0. 65 212 50 0. 03s V 0.0020.80 242 32 0. 048 stainless steel wire weft knit mesh having from l2 to18 A i needle ends per inch. These layers were stacked on top Examples33 to 35 of a perforated A181 304 stainless sheet having 0.028 diameterholes on 0.063 nch centers in an equilateral For appm-110005 requiringhigh Strength the resimertriangular pattern, sintered at 1,200C.,rolled, and sining step can be omitted, so that the knitted mesh c0mtered again. The final composite had a permeability of posite remains in awork-hardened condition, or heat 40 5O Rayls, a weight of L1 lbs/ft aYoungs modulus of treatable materials can be used. Example 33 is an in-13 X lO psi., a breaking strength of 12,240 lbs/ft. and V stance of theformer approach, Examples 34 and 35 the a specific strength of 1 1.12ft. V latter. These composites were prepared fromweft- By usingperforated sheet made of high strength alknitted wire mesh made of thesteel ll y noted in loys, for example, the precipitation hardeningalloys Table V, stacked using the number of sheets noted in 5 h as 17.7PH, 174 PH, AM 350, AM 35 5, etc the table, sintered at l,l-l,400C., andr ll d t th bonded to lower or higher yield strength knitted wirethickness noted in thetable. Examples 34 and 35 were mesh composites,even better mechanical properties resintered at l, 100-] ,250C., andheat treated for maxcan be obtained. imum strength followingmanufacturers published rec- Some high strength alloys are not readilyavailable, or ommendations. The Ray] number, breaking strength 50 arevery expensive, when made as fine wire. By comand specific strength werethen determined for all the bining perforated material made of very highstrength examples. alloys with knitted mesh composites made of readilyTABLE V Wire 7 Actual Breaking Specific diameter N0 of Heat- Weight Ray]thickness strength strength Example N0. Alloy (inches) layers Resinteredtreated (lb/ft!) No. (inches) (lbs/ft.) (ft.) 33 AISI 341". o. 004 0.287 0. 012 3,390 12,120 0. 004 0. 26 .10 0. 008 3, 912 15, 100 0. 004 0.27 s 0. 010 a, 660 13, 540

AM 355..-; RMCO 17-4PH These materials were quite satisfactory asfilters, and as acoustic absorption media.

It is quite remarkable that at a given Rayl Number and weight, thecomposites of the invention (compare Tables ll and Ill) are stronger inbreaking strength and specific strength than a woven wire fabric, andhave a higher modulus of elasticity. This means that the acousticinsulation or other acoustic unit can be smaller and available wire, ahigh strength product is obtained at low cost.

Example 41 V 19'. A commerciallyavailable welded stainless steelhoneycomb core made of0.004 stainless steel foil, 7/16 inch thick withit; inch cells, was placed between the two composites, and the sandwichsintered at l,200C.

between two flat plates. Adhesion of the facing sheets to the core wasfound to be satisfactory, and no mea surable loss in porosity of theknit mesh facing sheets was observed. The resultant structure wassuitable for sound'suppression and structural use.

Examples 42 to 46 v Honeycomb structures were made following theprocedure of Example 4l, except that only one face was a knit meshcomposite. Table VI lists the structures used.

TABLE VI Knitted Mesh Example Face per I 0. Example Other Face 42 60.003 inch solid stainless steel sheet 43 6 0.003 inch perforatedStainless steel sheet, 0.020 inch holes staggered. 237r open'area 4-1 70.003 inch perforated stainless steel sheet, 0.020 inch holes staggered,2371 open area 45 8 Sintered woven wire mesh, 58

Rayls. 0.0l7 inch thick, 0.24 lbs/ft". 4o 20 Sintered woven wire mesh,13

Rayls. 0.017 inch thick. 0.22 lbs/ft.

All structures showed good adhesion, no measurable reduction in porosityin the knitted mesh composite face, and were suitable for use in soundsuppression applications carrying structural loads.

The acoustic insulating structures of the invention comprise a framearranged along a passage for gas flow including laterally extendingchambers disposed behind a knitted mesh composite of the invention. Foreach sound wave ofa specific frequency there is an optimum chambervolume, to produce maximum sound attenuation. There also is an optimumvalue for the effective resistance of the anisometric, knitted wire meshcomposite at the entrance of the chamber. Each chamber and the knittedwire mesh composite at its entrance form a Helmholtz resonator, whoseacoustic impedance is equal to R +j.[(wM (l/mC)], where R flowresistance of the mesh composite M inductive reactance of the meshcomposite C capacitance of chamber volume pc /V p gas density speed ofsound V chamber volume to frequency, rad/sec. 2 11f The flow resistanceR is normally expressed in Rayls. The Rayl number is determined byinstalling the knitted wire mesh composite in the test apparatus shownin FIG. 8. The knitted wire mesh composite 50 is attached across theopen end of a plenum chamber 51, sealed against leakage by the rubbergaskets 52, in the jig 53. The plenum chamber is fitted with an airinlet line 54, connected to an air supply (not shown), with controlvalves 55, two pressure gauges 57, a pressure regulator 56, and a flowmeter 58. Another line 59 leads from the plenum chamber to a manometer60.

The Rayl number is determined by the equation:

- where AP pressure drop, dyne/cm r =jig radius, cm.

Q air flow, cm /sec. The Rayl number depends on the air flow rate usedfor the test, as shown in the curve of FIG. 9. Since high noise levelscorrespond to high air flows (60,000 ft/hr. corresponds to approximately160.5 db which is typical of aircraft engine noise levels), the Raylnumber should be measured at the sound intensity at which the knittedwire mesh composite will be used.

The normal incidence absorption coefficient and inductive reactance ofthe anisometric knitted wire mesh composite can be determined by testingin a standing wave tube apparatus, shown in FIG. 10. The loudspeaker 70at one end of the tube 71 is operated at the desired test frequency froman audio-frequency oscillator 72. The sound waves move through the tube7] and strike the sample 73 of knitted wire mesh composite which isplacedin a sample holder 74, which provides the desired volume behindthe sample. The sound waves are then partly reflected from the sample ofknitted wire mesh composite. The resultant of an incident wave withamplitude l and reflected wave with amplitude r is a standing wavepattern with alternate sound maxima l r and minima l r in the tube 71.From the ratio n of these sound pressure maxima'and minima thereflection coefficient r follows directly.

The sound field is explored by means ofa probe microphone 75 movable ona track 76 equipped with a scale 77 on which the exact distance betweenprobe entrance and test sample-of knitted wire mesh composite can beread. The microphone voltage is amplified by a selective amplifier 78 toreduce the influence of hum and noise and higher harmonics, which areinevitably generated by the loudspeaker in the tube.

FIG. 11 shows a typical curve of sound pressure vs. distance from theanisometric knitted wire mesh composite that is obtained using theapparatus of FIG. 10.

The distances y, and y are measured and the phase angle 0 of thereflection coefficient r is calculated by yr/y2) hr The impedance Z ofthe Helmholtz resonator is where r is a complex number. From thisequation and the magnitude and phase angle of r, and knowing in and C, Rand M can be calculated. I

Using the equations, it may readily be seen that, at

resonance which has a maximum at R pc. Since R varies with sound level,and the purpose of the knitted'wire mesh composite is to reduce thesound level, it is desirable to have the flattest possible curve of Rvs. air flow. A resonator fitted with a sound absorption facing mediumhaving a flat curve is efficient in absorbing sound at all sound levels,whereas use of a medium with a steep curve is efficient at one soundlevel only, hence overall sound attenuation is lower. To express thedegree of deviation from this ideal, the Rayl Curvature Factor has beendeveloped. This is the ratio of the Rayl number at 160.5 db (60,000ft/hr) to 134.5 db (3,000 ft/hr). The lower the Rayl Curvature Factor,the closer to ideal the composite is.

The above description applies to normal incidence sound absorption,where the theory is well known. When ducts are lined'with Helmholtzresonators so as to provide grazing incidence, with high air velocitiesand the spinning modes described by Tyler and Sofrin' in US. Pat. No.3,I98,487,'the theory is less well understood. This is probably becausethe impedance of the resonator is changed by the boundary layer in theflowing stream. However, it has generally been found that knitted wiremesh composites giving better performance in normal incidence tests willperform better in duct tests, with the exception that lower Rayl numbersare generally required because of the added impe dance of the boundarylayer.

Accordingly, FIG. 12 shows in cross-section a duct II, one wall of whichis lined by a honeycomb structure composed of a front face plate of ananisometric knitcomposite faces 34, opening first on one side and thenon the other of the structure, particularly useful when a splitter (i.e.a member with flow on both sides) is to be acoustically treated. Ofcourse, two structures as per FIGS. 12 or 13 can also be put back toback, to achieve. the same effect, but more space is then necessary forthe insulation.

The structure of FIG. 14 is arranged between two parallel passages 17and 18. The core comprises cells 28, 29 separated by the undulatingdivider 26. The cells 29 are closed at their ends 33 adjacent thepassage 18, and the cells 28 at their ends 33 adjacent the other passage17. The other ends of cells 28, 29 are faced with the knitted meshcomposites 34, which allow the passage of sound into the cells 28, 29.The sound is attenuated in this Way in both passages l7, 18 with asingle structure.

It is also possible to position the transverse walls of the structure inpairs at angles to one another, to form wedge-shaped cells, as in FIG.I5, the cells narrowing (converging) or widening (diverging) away fromthe passage, opening all on one passage, or on opposite sides, as inFIG..15. If they open only-on one passage,

ted wire mesh composite 13 of the invention, in this case, the compositeof Example 7, the honeycomb core, composed of cells 14, separated bydividers l2, and a solid back plate 16. The dividers separate the cells14 from each other, but they can also allow interconnection. Bestresults are usually obtained if no interconnection is provided. Theanisotropic character of the knit mesh composite is useful here, sinceside .flow through the composite which would provide someinterconnection between cells is substantially eliminated.

The dividers 12 may be metal ribs or partitions, or a honeycomb core ofresin impregnated paper or metal to which the knitted wire meshcomposite l3 and plate 16 are fastened by riveting, sintering, welding,brazing, resin-bonding, or the like. If the dividers 12 are metal andthe fastening method is sufficiently strong, the resultant structure maybe useful as an aircraft structural member, in addition to reducingmoise.

The exact Rayl number of the knitted wire mesh composite, volume depthand volume, and the total area required, are best determined empiricallyfor the desired installation.

The resultant installation will have a certain frequency range overwhich it will provide useful sound absorption. This range can beincreased by the double honeycomb structure shown in FIG. 13. Anadditional internal divider of anisometric knitted wire mesh compositel5 divides the honeycomb into two preferaby unequal honeycomb layers. Ifproperly proportioned, this structure provides two resonant frequencies,and attenuates sound over a wider band width than the structure shown inFIG. 12.

FIGS. 14 and 16 show configurations, with an undulatingdivider 26alternating serving as one face and then the other face of the structuredefining the closed cells 2 8, behind the anisometric knitted wire meshthe diverging cells can be tuned to lower frequencies than theconverging cells.

A simple zig-zag divider will create the wedge-shaped chambers.

In the form shown in FIG. 15, the transverse divider walls 37 of thestructure are made of a single zig-zag sheet touching, alternately, theopposed knitted mesh composite faces 34, so that the cells 30, 31 arealways closed at the points of the triangle against the penetration ofsound, and at the base of the triangle are connected by thesound-conducting knitted mesh composites 34 (represented in brokenlines) to the respective passage 38 or 39. The devices according toFIGS. 14 and 15 are also suitable for absorption-type acousticinsulation devices wherein the cells 28, 29 and 30, 31 are uniformlyfilled with loose absorbing material.

FIG. 16 shows a structure similar to FIG. 14 with the cells 28, 29opening into passages 20, 21, alternately, with knitted mesh composites34 permitting access to the cells, but with the soundimpermeable endwalls 22; 23 lined with sound-absorbing material 24.

FIG. 17 shows a honeycomb structure that is permeable to sound at eachface, having each face 13 of knitted wire mesh composite, and thehoneycomb core having transverse divider walls 12 extendingtherebetween.

FIG. 18 shows ahoneycomb structure like that of FIG. 17 with one face 13of knitted wire mesh composite and one face 19 of a perforated metalplate.

Acoustic insulating structures of these types are useful to attenuatejet engine noise When used to line the fan ducts, engine intake ductsand exhaust ducts ofjet engines. They are also useful in attenuatingsound when used as a lining for ventilating ducts for airconditioningand ternperature conditioning systems.

The anisometric knitted wire mesh composites can serve as acousticliners for gas ducts by merely disposing'cylinders (round, polygonal orother shape) thereof in the duct, concentrically or eccentrically, andsingly or in groups of two or more. The cylinders are so placed that gasflows along their surface and/or therewithin, in flowing through theduct. Why such an arrangement is effective in sound attenuation is notclear, buteyidently the open voids volume of the composite acts as aplurality of lateral passages with the surface sage of cool air to theinner surface, thus maintaining a cool layer of gas between the hotcombusted gases and the containing wall surface. This allows jet enginesto operate at higher temperatures without loss of chambcr wall strength.This function is called transpiration cooling.

Other important applications are in turbine blades and in the fuelinjection system of rocket engines. It has been shown that due to thevery high thermal conductivity of hydrogen; the heat from the combustionchabmer of a rocket engine can be transmitted to the hydrogen injectionnozzles, causing them to melt. Using transpiration cooling via ananisometric knitted mesh composite of the invention and cryogenichydrogen, this difficulty can be avoided. Other related applicationsare:

a. Use of porous media as an infra-red source by heating the porousstructure to over 2,000F. A typical application would be for stove typeradiators. b. Combustion chamber liners for helicopter blade tips, suchas a small porous combustion chamber liner for use in helicopter bladetip jet burners to prevent overheating c. Infra-red shielding. to. coolsurfaces below 600 B to prevent infrared radiation. a l

d. Supersonic and hypersonic wind tunnel nozzles. e. Transpirationcooling of hypersonic aircraft, re-entry and aerospace craft.

Having regard to the foregoing disclosure, the following is claimed asthe inventive and patentable embodiments thereof: 1

l An acoustically-insulated gas conduit comprising a conduit definingatleast one through-flow passage with laterally extending cells at oneside thereof, a plurality of spaced transverse walls separatingthe'cells one from anothenand an air-permeable conduit wall comprising aporous fluid-permeable anisometric composite comprising'a plurality oflayers of knitted wire mesh, compressed to a maximum pore diameter belowabout 200 microns, and having the wires lying almost entirely in planesapproximately parallel to the plane of the composite, and a voids volumeof at least IOpercent, the wires at the interface of the interior layersbeing intermirigled and interlocked with each other by such compression,and the wires and the layers: being bonded together at their points ofcontact, the composite being'connected to said transverse walls andextending across the cells in a manner to permit gas flow and sound toenter the cells from the passage.

2. An acoustically-insulated gas conduit in accordance with claim 1, inwhich the fluid-permeable composite has a thickness within the rangefrom about 0.001 to about 0.5 inch. I

3. An acoustically-insulated gas conduit in accordance with claim 1,wherein the wires within each layer and at the interface of the layersof the composite are sinter-bonded together.

4. An acoustically-insulated gas conduit in accordance with claim 1,wherein the layers of the fluidpermeable composite are of weft-typeknitted mesh.

5. An acoustically-insulated gas conduit in accordance with claim 1,wherein the layers of the fluidpermeable composite are of warp-typeknitted mesh.

6. An acoustically-insulated gas conduit in accordance with claim 1,wherein the layers of the fluid-- permeable composite are of knittedmesh having less than 30 needle ends per inch.

7. An acoustically-insulated gas conduit in accor dance with claim 1,wherein the fluid-permeable composite is made of stainless steel wire.

8. An acoustically-insulated gas conduit in accordance with claim 1, inwhich there are at least five layers of knitted wire mesh in onefluid-permeable composite.

9. An acoustically-insulated gas conduit in accordance with claim- 1,wherein the wires of the fluidpermeable composite are deformed at theirpoints of crossing, soas to have a lesser height and a greater width atthose points.

10. An acoustically-insulated gas conduit in accordance with claim 1, inwhichthe fluid-permeable composite has the wires sinter-bonded attheir'poin ts of crossing. r

11. An acoustically-insulated gas conduit in accordance with claim 1, inwhich the fluid-permeable composite has a modulus of elasticity of atleast 3.3 percent of the modulus of solid sheet of the same material.

12. An acoustically-insulated gas conduit in accordance with claim 1, inwhich the air-permeable conduit wall, laterally extending cells andtransverse walls enclose the through-flow passage.

13. An acoustically-insulated gas conduit in accordance with claim 1, inwhich the fluid-permeable composite has a layer of woven wire meshbonded thereto.

14. Anacoustically-insulated gas conduit in accordance with claim 1, inwhich the fluid-permeable composite has a layer of perforated metalsheet, bonded thereto. I i

15. An acoustically-insulated gas'c'onduit in accordance with claim 1,in which the air permeable conduit wall comprises portions of porouscomposite material and non-porous metal sheet material disposedalternatingly and across adjacent cells, respectively.

16. An acoustically-insulated gas conduit'in accordance with claim 1, inwhich the cells contain sound absorbent material.

17. An acoustically-insulated gas conduit in accordance with claim 1, inwhich cells and transverse walls comprise a plurality of honeycomb celllayers spaced by laterally extending dividers.

l -i =l

1. An acoustically-insulated gas conduit comprising a conduit definingat least one through-flow passage with laterally extending cells at oneside thereof, a plurality of spaced transverse walls separating thecells one from another, and an air-permeable conduit wall comprising aporous fluid-permeable anisometric composite comprising a plurality oflayers of knitted wire mesh, compressed to a maximum pore diameter belowabout 200 microns, and having the wires lying almost entirely in planesapproximately parallel to the plane of the composite, and a voids volumeof at least 10 percent, the wires at the interface of the interiorlayers being intermingled and interlocked with each other by suchcompression, and the wires and the layers being bonded together at theirpoints of contact, the composite being connected to said transversewalls and extending across the cells in a manner to permit gas flow andsound to enter the cells from the passage.
 2. An acoustically-insulatedgas conduit in accordance with claim 1, in which the fluid-permeablecomposite has a thickness within the range from about 0.001 to about 0.5inch.
 3. An acoustically-insulated gas conduit in accordance with claim1, wherein the wires within each layer and at the interface of thelayers of the composite are sinter-bonded together.
 4. Anacoustically-insulated gas conduit in accordance with claim 1, whereinthe layers of the fluid-permeable composite are of weft-type knittedmesh.
 5. An acoustically-insulated gas conduit in accordance with claim1, wherein the layers of the fluid-permeable composite are of warp-typeknitted mesh.
 6. An acoustically-insulated gas conduit in accordancewith claim 1, wherein the layers of the fluid-permeable composite are ofknitted mesh having less than 30 needle ends per inch.
 7. Anacoustically-insulated gas conduit in accordance with claim 1, whereinthe fluid-permeable composite is made of stainless steel wire.
 8. Anacoustically-insulated gas conduit in accordance with claim 1, in whichthere are at least five layers of knitted wire mesh in onefluid-permeable composite.
 9. An acoustically-insulated gas conduit inaccordance with claim 1, wherein the wires of the fluid-permeablecomposite are deformed at their points of crossing, so as to have alesser height and a greater width at those points.
 10. Anacoustically-insulated gas conduit in accordance with claim 1, in whichthe fluid-permeable composite has the wires sinter-bonded at theirpoints of crossing.
 11. An acoustically-insulated gas conduit inaccordance with claim 1, in which the fluid-permeable composite has amodulus of elasticity of at least 3.3 percent of the modulus of solidsheet of the same material.
 12. An acoustically-insulated gas conduit inaccordance with claim 1, in which the air-permeable conduit wall,laterally extending cells and transverse walls enclose the through-flowpassage.
 13. An acoustically-insulated gas conduit in accordance withclaim 1, in which the fluid-permeable composite has a layer of wovenwire mesh bonded thereto.
 14. An acoustically-insulated gas conduit inaccordance with claim 1, in which the fluid-permeable composite has alayer of perforated metal sheet, bonded thereto.
 15. Anacoustically-insulated gas conduit in accordance with claim 1, in whichthe air permeable conduit wall comprises portions of porous compositematerial and non-porous metal sheet material disposed alternatingly andacross adjacent cells, respectively.
 16. An acoustically-insulated gasconduit in accordance with claim 1, in which the cells contain soundabsorbent material.
 17. An acoustically-insulated gas conduit inaccordance with claim 1, in which cells and transverse walls comprise aplurality of honeycomb cell layers spaced by laterally extendingdividers.